refactor(oauth): rename credentials/ to oauth/ for clarity

Separate OAuth credential reading (external CLI tools) from the main
CredentialManager (API Key configuration in credentials.json5).

- credentials.ts: API Key configuration management
- oauth/: External OAuth credential reading (Claude Code, Codex)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/cli/commands/chat.ts

@@ -17,7 +17,7 @@ import {
   getCurrentProvider,
   getLoginInstructions,
   type ProviderInfo,
-} from "../../credentials/providers.js";
+} from "../../oauth/providers.js";
 
 type ChatOptions = {
   profile?: string;

src/agent/oauth/providers.ts

@@ -36,13 +36,13 @@ export interface ProviderInfo {
 
 export interface ProviderConfig {
   provider: string;
-  model?: string;
-  apiKey?: string;
-  baseUrl?: string;
+  model?: string | undefined;
+  apiKey?: string | undefined;
+  baseUrl?: string | undefined;
   // OAuth specific
-  accessToken?: string;
-  refreshToken?: string;
-  expires?: number;
+  accessToken?: string | undefined;
+  refreshToken?: string | undefined;
+  expires?: number | undefined;
 }
 
 // ============================================================

src/agent/runner.ts

@@ -7,7 +7,7 @@ import { SessionManager } from "./session/session-manager.js";
 import { ProfileManager } from "./profile/index.js";
 import { SkillManager } from "./skills/index.js";
 import { credentialManager, getCredentialsPath } from "./credentials.js";
-import { resolveProviderConfig } from "./credentials/providers.js";
+import { resolveProviderConfig } from "./oauth/providers.js";
 import {
   checkContextWindow,
   DEFAULT_CONTEXT_TOKENS,